1. Field of the Invention
The invention relates to the field of the thermal treatment of waste. It refers to a method for the incineration of refuse in an incineration furnace and for treating the slag from this refuse incineration.
2. Discussion of Background
The grate firing method is generally used currently for the incineration of domestic refuse. In this method, the refuse is moved mechanically over a horizontal or inclined plane and at the same time combustion air, which enters the bed of refuse from below through the grate, flows through the refuse. The incombustible fraction of the waste is removed from the incineration plant as grate ash or slag. While the grate firing method can be employed to excellent effect for refuse having a calorific value of over 6500 kJ/kg, it is not suitable for the incineration of refuse with lower calorific values, since in this case a high level of combustion-air preheating is required to dry the refuse, leading to disadvantageous strength and corrosion problems with the grate equipment.
EP 0 372 039 B1 has disclosed a method for treating the slag from waste incineration plants in which the slag is removed from the incineration furnace in the dry state, is subjected to crude cleaning (removal of unburnt coarse material and magnetic components), and then the crudely cleaned slag is separated into at least two fractions, and all the particles which are smaller than 2 mm are assigned to one fraction. This method is based on the knowledge that the fine fraction contains most of the pollutants which were originally contained in the slag. The fine fraction is fed to a special treatment, while the coarse fraction is suitable, for example, as a construction material.
A further development of this method is disclosed in EP 0 722 777 A1, which claims a method for the treatment of slag from refuse incineration plants in which the crude slag, after having passed through the firing grate, is immediately separated into at least two fractions, without prior quenching in a water bath, and these two fractions are treated further separately, the coarse fraction being fed to a wet deslagger. In the method, the first fraction, with a particle size of smaller than 80 mm, preferably smaller than 32 mm, is separated off in a first screening stage, the screen overflow is fed to wet deslagging, the screen underflow and, if appropriate, the grate underflow from the firing grate, is fed to a second screening stage for the purpose of separating the fine fraction having a particle size of up to 2 mm, the screen overflow from the second stage is mechanically comminuted, if appropriate after metallic and inert materials have been sorted out, and the screen underflow from the second stage is fed to a special treatment, e.g. a melting furnace. In this melting process, which is carried out, by way of example, in an arc furnace, a vitreous product which is readily suitable for landfill and a mental concentrate are produced (cf. F. -G Simon and K. -H. Andersson: InRec-Verfahren--Verwertung von Reststoffen aus der thermischen Abfallbehandlung [InRec process--Utilization of residual materials from the thermal treatment of waste], ABB Technik September 1995, pp. 15-20). This treatment method has thus far been put into practice for the slag from grate incineration furnaces, where it has proven useful. The high costs caused by the use of the arc furnace represent a disadvantage.
In addition to the grate incineration method for refuse, it is also known to incinerate refuse (predominantly special refuse) in a drum-type furnace. Drum-type furnaces essentially comprise a cylinder which is inclined in the direction of conveyance and is lined on the inside with refractory material or has a cooled hollow jacket. The refuse, which may be of differing consistency and condition, is fed to the drum at the top end of the drum in the co-current method, together with the combustion air, and is then incinerated in the drum. A drawback of this method is that the combustion air cannot flow through the bed of refuse, and as a result a poor slag burn-off is achieved. Although this can be avoided by prolonging the residence time of the refuse or by increasing the temperature in the drum, this in turn has the following drawbacks: the first measure leads either to a large drum-type furnace or to a low throughput, and the second measure leads to the slag melting, entailing high wear to lining material and hence to high treatment costs. For these reasons, in practice generally only special refuse is incinerated in the drum-type furnace, which refuse, owing to its varying consistency, cannot for its part be treated in the grate furnace.
For waste materials with a low calorific value and a particularly high water content, it is advantageous to employ the counter current principle in the drum-type furnace, i.e. the waste-slag path runs in the opposite direction to the combustion air/flue gas path. In this case, the vapors from the drying zone are diverted directly into the combustion chamber without affecting the rest of the firing area of the drum-type furnace. As a result, the waste materials become ignited throughout more quickly, and a shorter drum is sufficient for complete burn-off (K. J. Thome-Kozmiensky: Thermische Abfallbehandlung [Thermal Treatment of Waste], EF-Verlag fur Energie- und Umwelttechnik BmbH, second edition, 1994, p. 240).
A further drawback of using drum-type furnaces for the incineration of refuse consists in the fact that the mixing between slag and air therein is poor. Drum-type furnaces are operated with a high excess of air from 2.0 to 3.0 (K. J. Thome-Kozmiensky: Thermische Abfallbehandlung [Thermal Treatment of Waste], EF-Verlag fur Energie- und Umwelttechnik GmbH, 2nd edition, 1994, p.239), leading to high NOx values which, however, ought to be kept as low as possible for environmental protection reasons.